During digestion, the body breaks down the carbohydrates in the food we eat into glucose and then releases the glucose into the bloodstream. Body tissues, such as the brain, use glucose as a vital energy source. Carbohydrates provide the quickest source of energy for your body, but depending on the type of carbohydrate that you have consumed (simple or complex, refined or natural), it may be a short-lived source of energy. (See, "Carbohydrate Classifications.")
If you have consumed more carbohydrates than your body needs, the glucose will be turned into glycogen and stored in your muscles and liver for future use. When the liver and muscle tissues exceed their capacity for storing glycogen, the excess is converted into fat. This is why proponents of a low carbohydrate lifestyle maintain that if you reduce your intake of carbohydrates, your body will not produce excess glycogen; therefore, your body will not create additional fat.